The generation of an H2/CO synthesis gas is done—among other things—by reforming of methane and other light hydrocarbon loads with steam (steam methane reforming or SMR), at very high temperature. Several reactions occur during this reforming step and, though some are exothermic, the initial and main reaction is endothermic, so that it is necessary to provide heat for this step of generating synthesis gas (or syngas).
Various fuels are normally burnt to provide this heat for the reforming, including a so-called primary fuel—typically a mixture of light hydrocarbons ranging from natural gas to naphtha, frequently a fraction taken from the feed gas—and a so-called secondary fuel, consisting at least partly of one or more residual gases resulting directly or indirectly from the method of treating the synthesis gas produced, —in particular, in the event of a final production of hydrogen from the synthesis gas produced, the secondary fuel will contain all or part of the residual gas (or offgas) from the hydrogen purification unit, normally a pressure swing adsorption unit (PSA). These primary and secondary fuels are burnt in the combustion area of the reformers (SMR) in order to provide the heat necessary for reforming. The temperature levels required in the reforming area are very high and, because the temperature at which reforming occurs is above 800° C. (and in general above 850° C.), a large quantity (typically between 40% and 60%) of the heat released during combustion is not used for the reforming. In fact, only the proportion of heat released during the combustion and having a sufficient temperature to provide heat to the reforming process is useful for the reaction. The rest of the heat released by the combustion must be reused in the overall process in order to obtain an acceptable overall thermal efficiency. This proportion of the heat of the combustion that is not used by the reforming reaction is situated in the fumes issuing from the reforming area (radiation) and is recovered in the convection area. This recovery of heat takes place from temperature levels that may reach 1080° C. to a lower temperature limit corresponding to the temperature below which the sending of the fumes to atmosphere is acceptable (around 180° C. or less according to current standards).                The heat available in the fumes can thus cover various requirements, in particular: preheating of the hydrocarbon load;        “steam generation”, meaning—by extension—the heating of liquid water or water vapour, and the preheating of demineralised water (DMW) and so-called boiler feed water (BFW), vaporisation and superheating of steam;        preheating of combustion air (or exported process air).        
The various preheatings take place via exchangers present in the convection area where the fumes circulate. The fumes that have been cooled during the various preheatings are next released into the atmosphere via a blower and/or a flue. The quantity of heat available for these various heatings depends on the temperature of the fumes, but also obviously on the flow rate thereof.
During combustion, the fuels (primary and/or secondary) are burnt in the presence of oxidants (also referred to as oxidising agents); air (which may be depleted of or enriched with oxygen), an oxygen provider, is typically used as an oxidant.
The air may be air depleted of oxygen (but at least 10% O2 in air), or enriched with oxygen (up to 40% O2 in air).
The air must always be in excess—with respect to stoichiometry of the combustion reaction—so that all the fuel gases are consumed, but in limited excess. It is sought to adapt the quantity of air so as to keep the excess of air to a level of 5% minimum, without exceeding 40% with respect to the requirements of the stoichiometry of the combustion, preferably between 5% and 20% with respect to stoichiometry.
Providing a limited excess of air makes it possible:                firstly to adapt the consumption of fuel (PSA residual gas and primary fuel) to the requirements for heat, both for the reforming reaction in the radiation area (heat necessary for achieving a high temperature, above 1000° C.) and for the various preheatings in the convection area (hydrocarbons, DMW, BFW, generation and superheating of steam, preheating of air, etc.),        secondly to limit the concentration of excess oxygen that is situated in the fumes; the oxygen content of the fumes must in fact remain limited; the oxygen concentration of the fumes in the convection area (fume chain) must remain less than the minimum concentration of oxygen required for ensuring the combustion of any hydrocarbons present (minimum oxygen concentration or MOC)—the expression MOC will be used in the remainder of the description to mean “minimum concentration of oxygen required for ensuring combustion”. Observing this precaution means that, in the event of a leakage of hydrocarbons, at a heat exchanger for preheating the hydrocarbon load for example, the oxygen content is too low for the hydrocarbons issuing from the leakage and present in the convention area to ignite.        
Moreover, current constraints, in particular economic and environmental, mean that frequently some of the compounds inert towards combustion, which were normally present in the offgas of the PSA, are taken off (before or after the PSA) but in all cases prior to the recycling of the stream or streams concerned to the reforming burners. This is in particular a consequence of the extraction of CO2 via the direct liquefaction of the offgas of the PSA or the primary decarbonation of the synthesis gas via the MDEA, before the PSA.
However, if the inert compounds extracted from the secondary fuel do not contribute to combustion, their presence does however require additional fuel for heating them up to the temperature level necessary for a flame temperature enabling the reforming action to take place with suitable efficiency. Their presence in the fumes contributes to the volume thereof and represented a large quantity of heat usable in the convention area; a reduction in the quantity of inert compounds in the gaseous mixture sent to the burners therefore signifies reduced heating requirements, a reduced requirement for fuel, and therefore a requirement for oxidant, that is to say air, that is also reduced.
In this case, maintaining the excess air within the limits required for the oxygen content in the fumes to remain below the MOC also contributes to reducing the total mass flow of the fumes.
It can therefore be seen that, because of the capture of the CO2 previously present in the residual PSA, the reforming fumes are reduced not only with respect to the CO2 coming from the residual gas but also with respect to a certain quantity of inert substances brought by the air, because of the reduction in the quantity of air necessary for heating the gases present during combustion.
A first consequence of the reduction in the volume of the fumes is that, the prime function of the combustion being to provide for the requirements of the reforming reaction, the reduction in the contribution of heat to the tubes by the combustion gases emitted in the combustion chamber will be compensated for by a suitable increase in the supply of oxidant (air) and by a greater makeup of primary fuel.
Nevertheless, the quantity of heat available in the convection area of the SMR is greatly reduced and the contribution of heat necessary for the various preheatings normally provided in the convection area is no longer assured. A first problem then lies in the fact that it is no longer possible to maintain the preheating temperatures and/or the quantities of fluids preheated in the convection area such as they were assured when the inert substances were present in the fumes. This reduction in the preheating possibilities via the fumes may be detrimental to the reforming process itself (reduction in the efficacy of the reforming because of insufficient preheating of the various reagents, and reduction in the production of steam). A second problem then stems from this, the heat exchangers present in the convection area being designed with given operating ranges, depending on the quantity of inert compounds taken off, the process may be situated outside suitable ranges, which will require expensive modifications to them.
It is therefore important, in order to avoid the aforementioned drawbacks, to be able to maintain the quantity of transferable heat in the convection area of an SMR furnace in the event of removing inert substances from a gas supplying the combustion of said SMR furnace; in particular, in the event of capture of the carbon dioxide normally present in the residual PSA gas, it contains, in the absence of capture of CO2 upstream of the PSA, a large proportion of CO2 of around 40% or more.